Field of the Invention
This invention relates generally to systems for correcting a variable magnitude input signal which represents an input parameter and which is subject to undesired offsets and gain variations in its representation of that parameter; and more particularly, relates to such systems for use in scanning type facsimile transmission and reception equipment to correct effects and gain variations in the video signal which represents the shade of darkness or lightness (i.e., shade intensity) of source document areas being scanned.
In many facsimile transmission and receiving systems a source document, whose contents are to be transmitted from a facsimile transmitter to a remote facsimile receiver and reproducer, is scanned line by line by a narrowly focused light beam which illuminates the document areas being scanned. The light reflected from the scanned document areas is collected by a suitable photosensor which is positioned adjacent the document to produce a corresponding video signal whose voltage at any instant is representative of the shade of darkness or lightness of the document area being scanned at that instant. This video signal is then applied as one input signal to a suitable transmit modulator, which also receives a variety of synchronizing pulses provided by other elements of the facsimile transmitter and operates to transmit both the video information and the synchronizing signal information by a suitable modulation system (as for example, frequency modulation or PCM, etc.) over a transmission path such as phone lines or a radio transmission path to a remote facsimile receiver.
The signals which are received over the transmission path by the remote facsimile receiver, are first processed in a suitable demodulator, which extracts both the video portion of the signal and also the synchronizing pulse portions of the signal, and produces a corresponding video input signal and synchronizing pulse signals. The video signal is applied as an input to writing means in the facsimile receiver and may be applied, for example, to control the intensity of a light beam which writes upon photosensitive recording paper to reproduce thereon the image being scanned by the reading light beam of the facsimile transmitter. The synchronizing pulses extracted by the demodulator of the receiver are applied to the writing apparatus of the receiver to control the positioning of the writing means and may be applied, for example, to the vertical and horizontal deflection controls of a light beam producing unit to synchronize the horizontal and vertical scans of the writing beam in the receiver so as to illuminate at any time recording areas corresponding to the source document areas then being scanned in the transmitter.
One facsimile transmitting and receiving system, a transceiver employing a cathode ray tube flying spot scanner as the light beam forming unit for both illumination of the source document during transmission operation and writing of the copy document during receiver operation, is described in detail in co-pending application, Ser. No. 312,114, entitled "Graphic Indicia Video Signal Acquisition Transmission and Reproduction System", filed Dec. 4, 1972 by John S. Campbell, et al and assigned to the assignee of the present application now U.S. Pat. No. 3,843,839. Said application, Ser. No. 312,114, is a continuation-in-part of application 219,249, filed Jan. 20, 1972 by Campbell, et al.
In the transmitter of any such facsimile system, the video signal produced by the photocell sensor, which is monitoring the light reflected by the source document from the scanning light beam, is subject to voltage offsets and gain variations which vary during the day and from day to day in accordance with such factors as variations in the intensity of the illuminating light beam; drift of bias and sensitivity of the photosensor due to aging or temperature change or voltage variations, etc.; and bias and gain drifts in the preamplifier ordinarily associated with the photocell or photodiode; and many other factors.
All the factors which contribute to the general drift of offset and amplitude in the video signal, can be individually controlled in manners well known to the art but only by use of precise and relatively expensive circuit elements in the individual circuits relating to each factor. Thus, overall in the transmitter, the use of an uncorrected video signal results in placing greater requirements for precise and accurate circuitry and resulting expense.
In a facsimile receiver the very same problem is presented in a more acute form, for in a receiver, if the video signal extracted by the receiver demodulator is not corrected for undesired voltage offsets and gain variations, then such perturbations will, when the video signal is applied to control writing beam intensity, be directly reflected in corresponding perturbations of intensity of the recording light beam, resulting in undesired changes in the shade of the supposedly white background of the recorded or reproduced document and undesired changes and variations of the range of dark and light shades (i.e., contrast) of the image portions of the document. Thus, in a facsimile receiver, failure to eliminate or correct drift of offsets and amplitude variations of the video signal results in reduction of accuracy of reproduction of both background and contrast in the recorded or reproduced document image.
In addition, if correction of the extracted video signal produced by the receiver's demodulator is not available, then it becomes necessary to use more precise and expensive circuitry in the demodulator. In a receiver, drift of offset and amplitude of the extracted or detected video signal is caused by the factors of, drift in time delays in the transmission path, and drift in the voltages and components of the demodulator circuitry. Both factors can be compensated for by careful design of the receiver demodulator with precision components and circuitry, but the expense of the demodulator is correspondingly raised.